A liquid crystal display (LCD) device includes and LCD panel and a backlight unit including a light emitting diode (LED) light bar and an LED backlight driving circuit. Each of LED light bars is formed by a plurality of LED lamps. When a number of the LED lamps increases, an output voltage outputted by the backlight driving circuit accordingly increases, where the output voltage is usually greater than 100V, which requires use of an isolated boost circuit, as shown in FIG. 1A and FIG. 1B. A driving signal is used to control a controllable switch Q1 to turn on/off, where the controllable switch Q1 is a metal-oxide-semiconductor field-effect transistor (MOSFET). A transformer T is used to increase the output voltage outputted by the backlight driving circuit, where a ratio of a primary coil turn and a secondary coil turn of the transformer T is 1:N. If an input voltage of the transformer T is Vin, and an output voltage of the transformer T is Vo, an equation of the output voltage of the transformer T is: Vo=Vm*N*D/(1−D).
After the controllable switch Q1 turns off, a voltage of a drain electrode of the controllable switch Q1 is great, when the controllable switch Q1 turns on again, a power loss of the controllable switch Q1 is great. Additionally, temperature of the controllable switch Q1 increases, which shortens working life of components.